The structural and engineering uses of polymeric materials continues to grow rapidly in areas as diverse and protective paint coverings, wire insulations, structural panels for automobiles and piping of all kinds. In such cases the stability of the polymer is of paramount importance. The hydrogenation of olefin polymers, and especially diene-containing polymers such as butadiene-styrene copolymers, and the like, yields substances of greater saturation and consequently greater stability when used for certain applications. Rheological additives, for example, viscosity index improvers, are one class of compounds produced by the saturation of diene polymers.
The prior art has been interested in and examined polymer hydrognation processes for many years (U.S. Pat. No. 2,8 64,809), as a method to prepare novel materials with some unusual properties. For the most part, the early processes emphasized heterogeneous catalysts commonly used for saturation of low molecular weight olefins and aromatics, such as the nickel on kieselguhr system. A fine catalyst powder was preferred and large amounts of catalysts were required to complete the hydrogenation in a reasonable time. Such processes were only partially successful, since the reaction requires the diffusion of large polymer molecules into the pores of the catalyst, where the active nickel metal is present. This is a slow process with large molecules. The discovery of the nickel octoate/tiethyl aluminum catalyst system led to rapid hydrogenation of the polymer dissolved in a saturated solvent. Such a process was used for a number of years to prepare hydrogenated butadiene-styrene polymers that are used as viscosity-index improvers in premium motor oils (U.S. Pat. No. 3,554,991). There are no pore diffusion problems with homogeneous catalysts and the hydrogenation process is rapid and complete in a matter of minutes. However, the removal of the catalyst from the polymer soluton requires the additon of an ammonium phosphate-water soluton and air to oxidize the nickel to a divalent state. The mixed nickel-aluminum phosphate can be removed from the hydrogenated polymer solution by filtration. Many of the advantages associated with the use of homogeneous hydrogenation catalyst systems are offset by the catalyst removal step (U.S. Pat. No. 3,531,448), particularly when disposal of the spent catalyst is considered as part of the operation. We and others have repeatedly looked for suitable olefin polymer hydrogenation catalysts, particularly catalysts that could be used in fixed bed reactors. Recently we have discovered a commercial material that is a suitable catalyst support for use in fixed bed reactors.
There is a continuing effort to develop improved hydrogenation catalysts and processes to selectively hydrogenate polymers, such as olefin polymers, and especially diene-containing polymers, to hydrogenated polymeric products having desirable properties. The present invention is directed to a catalyst system and process for hydrogenating olefin polymers, especially diene-vinyl aromatic copolymers, to form polymers having reduced unsaturation.